Reflection type liquid crystal display element, display unit, projection optical system, and projection display system

ABSTRACT

A superior reflex type vertically-aligned liquid crystal display device wherein the refractive index anisotropy Δn of its liquid crystal material is controlled to be more than 0.1, and the transmissivity of the liquid crystal is saturated with facility at a low voltage below 5 to 6V despite a reduction of the thickness of the vertically-aligned liquid crystal layer to less than 2 μm, hence achieving satisfactory driving at a practically low voltage while attaining another advantage of remarkable improvement in the transmissivity itself. Therefore, the display device indicates a sufficient transmissivity, an excellent low-voltage driving characteristic and a fast response. Further improvements are realizable in a display apparatus, a projection optical system and a projection display system by the use of such display device.

BACKGROUND OF THE INVENTION.

The present invention relates to a reflex liquid crystal(electro-optical) display device adapted for a projection display systemor the like, and also to a display apparatus, a projection opticalsystem and a projection display system used in combination with such adisplay device.

With the recent progress in realizing improved projection display with ahigh definition, a small size and a high luminance, there are noted andpractically utilized reflex display devices which are suited forachieving a dimensional reduction with an enhanced definition and arecapable attaining a high optical efficiency.

Out of such display devices, there is reported an active reflex liquidcrystal display device wherein a driving element is provided on asilicon substrate which is positioned opposite to a glass substratehaving a transparent electrode formed therein and is composed of, e.g.,a CMOS (complementary metal oxide semiconductor) circuit, and a drivingcircuit substrate having an aluminum optical reflecting electrode isplaced thereon, and a vertically-aligned liquid crystal material isinjected between the pair of such substrates (Paper (1): H. Kurogane etal., Digests of SID1998, p. 33–36 (1998); Paper (2): S. Uchiyama et al.,Proceedings of IDW2000, p. 1183–1184 (2000)). The devices of this typehave practically been commercialized by some makers.

Here, the vertically-aligned liquid crystal material is one having anegative permittivity anisotropy (i.e., Δε (=ε(∥)−ε(⊥), which is thedifference between the parallel permittivity ε(∥) and the verticalpermittivity ε(⊥) to the major axis of the liquid crystal molecule, isnegative). When the voltage applied between its transparent electrodeand light reflecting electrode is zero, the liquid crystal molecules areoriented to be substantially vertical to the substrate plane to therebygive display in a normally black mode.

The thickness (cell gap) of the vertically-aligned liquid crystal layerin the conventional reflex device reported in the above theses is 3 to 4μm, and the curve of the liquid crystal transmissivity to the drivingvoltage applied to the liquid crystal (hereinafter referred to as V-Tcurve, which corresponds to the reflectivity of the device measuredactually in the reflex device; it is supposed here that the incidentlight, e.g., s-polarized light, is modulated into p-polarized reflectedlight by the device as will be described later) has such characteristicthat it rises at a threshold voltage of 2V or so and reaches its maximumvalue at an applied voltage of 4 to 6V. The transmissivity of the liquidcrystal is changed analogously by changing the voltage between theelectrodes to thereby realize expression of gradations. FIG. 14graphically shows data excerpted as an example from Paper (1) citedabove. According to the reported data, the liquid crystal layer has athickness of 3 μm, the driving voltage is approximately ±4V, and theresponse speed (rise time+fall time) is 17 msec or so.

Normally the liquid crystal is driven while the voltage is inverted tobe positive or negative per frame or field, so that the above device isactually driven by a voltage of ±4 to 6V at the maximum. (Since thepositive and negative V-T curves are mutually symmetrical in principle,it is usual that the V-T curve is expressed as positive alone.) It isalso defined that a liquid crystal driving voltage of ±4 to 6V needs tobe more than 8 to 12V as an effective withstand voltage of a drivingtransistor.

Since this voltage is considerably higher than the with stand voltage ina normal MOS process, a high withstand voltage process for an LDD(lightly doped drain-source) structure or the like is applied to aliquid crystal driving transistor formed in each pixel on the silicondriving circuit substrate. Considering the production cost, powerconsumption and so forth, the withstand voltage is generally in a rangeof 8 to 12V. This is the reason that the known device is so designed asto have a V-T curve of ±4 to 6V at the maximum.

In the vertically-aligned liquid crystal material used in the knowndevices, the refractive index anisotropy Δn (=n(∥)−n(⊥), which is thedifference between the refractive index n(∥) along the major axis of theliquid crystal molecule and the refractive index n(⊥) vertical thereto),has a value less than 0.1 (typically 0.08 or so), and the typical pixelpitch is 13.5 μm (pixel size 13 μm).

Recently, one defect of the liquid crystal display device concerning alow response speed thereof is attracting attention as a problem, and itis well known that raising the response speed is an important requisite.In general, the response speed (rise time and fall time) of the liquidcrystal is proportional to the square of the thickness d of the liquidcrystal layer, as expressed by Eq. (1) and Eq. (2) below. Therefore,reducing the thickness of the liquid crystal layer is effective toattain a higher response speed.

$\begin{matrix}{{{rise}\mspace{14mu}{{time}:{\tau\;{on}}}} = \frac{\gamma \cdot d^{2}}{{ɛ(0)}\Delta\;( {V^{2} - {Vc}^{2}} )}} & (1) \\{{{fall}\mspace{14mu}{{time}:{\tau\;{off}}}} = \frac{\gamma \cdot d^{2}}{K \cdot \pi^{2}}} & (2)\end{matrix}$(where γ: viscosity of liquid crystal, d: thickness of liquid crystallayer, Δε: permittivity anisotropy of liquid crystal, ε(0): spacepermittivity, K: elastic constant of liquid crystal, V: voltage appliedto liquid crystal (liquid crystal driving voltage), Vc: thresholdvoltage).

However, in the vertically-aligned liquid crystal display device knownheretofore, although the response speed thereof is rendered higheraccording to Eqs. (1) and (2) with reduction of the thickness of theliquid crystal layer, there arises another problem that the drivingvoltage required for saturating the transmissivity becomes higher. FIG.15 graphically shows V-T curves obtained by reducing the thickness ofthe liquid crystal layer in a system using a liquid crystal material(where Δn=0.082) employed in a conventional device, and FIG. 16graphically shows changes caused in the saturation voltage with thethickness d of the liquid crystal layer.

As shown in FIGS. 15 and 16, the saturation voltage of the devicebecomes sharply higher over 6V after the thickness d of the liquidcrystal layer is reduced to less than 2.5 μm, and the saturation voltagereaches high as 10V or so when the thickness d is less than 2 μm. Thatis, the withstand voltage required for the driving transistor needs tobe higher than 20V. In addition, when the thickness d is less than 1.5μm, the absolute value of the transmissivity fails to reach 100%. Incase the thickness d is 1 μm, the transmissivity attainable is merely30% or so, while the threshold voltage is raised to be higher.

Such a phenomenon is considered to result from that, with a reduction ofthe thickness d (cell gap) in the vertically-aligned liquid crystal, theinteraction exerted on the interface between the liquid crystalmolecules and the orientation film becomes relatively greater to thedirectional change caused in the director of the liquid crystalmolecules by the applied voltage. To the contrary, when the liquidcrystal layer has a sufficient thickness, the director is rendered moremobile due to the property as a bulk, whereby the interaction on theinterface is decreased in effect.

As described above, if the driving voltage becomes higher in the liquidcrystal display device, proper driving fails to be performed in anordinary driving device substrate of silicon. It is a matter of coursethat this problem can be solved by raising the withstand voltage of thepixel driving transistor, but generally the process is complicated withfurther disadvantages of increasing both the production cost and thepower consumption. Further due to such a rise of the withstand voltage,it is unavoidable that the transistor size is enlarged. For this reason,it becomes extremely difficult to manufacture high withstand-voltagetransistors in a small pixel size (or pitch) less than 10 μm or so inparticular.

For the reason mentioned above, it is practically difficult, in anyknown reflex display device using the conventional vertically-alignedliquid crystal, to reduce the thickness of the liquid crystal layer toless than 2.5 μm.

Reducing the thickness of the liquid crystal layer as described slowsdown the rise (response speed) to the applied voltage and lowers theyield in manufacture of the device.

Further, in any projection optical system equipped with such knowndisplay device, the F number of the optical unit needs to be equal to orgreater than 3.5 for maintaining a high contrast as will be explainedbelow, hence bringing another problem that a high luminance is notattained.

In any projection system equipped with reflex liquid crystal displaydevices, as shown in. FIG. 17, there is required an optical unit whereinluminous flux emitted from a lamp light source 1 is irradiated to reflexliquid crystal display devices 3R, 3G, 3B, each using vertically-alignedliquid crystal, via polarized beam splitters 2R, 2G, 2B which serve aspolarized light separating devices for red (R), green (G) and blue (B)respectively, and the reflected light beams modulated by such devicesare collected by a prism (X-cube prism) 4 which synthesizes the lightbeams of the individual colors, and then the composite light beam isprojected as projection light 10(p) to an unshown screen via aprojection lens 5.

Here, in an illumination optical unit for illuminating the reflex liquidcrystal devices 3R, 3G, 3B, the white light (10(p,s) composed ofp-polarized component and s-polarized component) from the white lamplight source 1 is processed to be s-polarized light 10(s) via a fly-eyelens 6, a polarizer/converter 7, a condenser lens 8 and so forth.Subsequently the s-polarized light 10(s) is introduced to a dichroiccolor separation filter 9, and the light separated therethrough is sentto total reflection mirrors 11, 12 and a dichroic mirror 13 toconsequently become light 10R(s), 10G(s) and 10B(s) of individualcolors. Thereafter the light is incident upon the reflex liquid crystaldisplay devices 3R, 3G, 3B respectively via the polarized beam splitters2R, 2G, 2B, and the reflected light is polarized and modulated inaccordance with the voltage applied to the reflex liquid crystal displaydevices 3R, 3G, 3B. After incidence upon the polarized beam splitters2R, 2G, 2B again, only the p-polarized components 10R(p), 10G(p), 10B(p)of the light are transmitted and then are synthesized by the prism 4.Consequently, when the applied voltage is zero in the reflex liquidcrystal display device, the incident light is reflected directly ass-polarized light without passing through the polarized beam splitter,and thus the system is placed in a normally black mode where the lightis polarized and modulated with a rise of the applied voltage, so thatthe p-polarized reflected light is increased to eventually raise thetransmissivity (refer to FIG. 14).

In the optical unit employed for the known vertically-aligned liquidcrystal display device reported in Papers (1) and (2), the F number isequal to or greater than 3.5 (e.g., 3.8 to 4.8 in Paper (1), or 3.5 inPaper (2)). The F number of the optical unit is a function of theincidence angle (outgoing angle of reflected light) θ of the lightincident upon the device, and it is expressed as follows.F=1/(2×sin θ)  (3)

An expression of F=3.5 signifies that the device face is illuminated bythe light within an angle of θ=±8.2° centering around a lineperpendicular to the device face, and the reflected light is obtainedtherefrom.

As obvious from Eq. (3), the smaller the F number, the light incidenceand outgoing angle θ become greater to consequently increase the totalluminous flux, hence raising the luminance. However, in the reflexliquid crystal device, generally the black level value (transmissivityin a black state) becomes higher with an increase of the incidenceangle, and the polarized-light separation characteristic of thepolarized beam splitter is dependent on the angle θ, whereby it isunavoidable that the characteristic is deteriorated with an increase ofthe angle θ, and the degree of separation into the p-polarized lightcomponent and the s-polarized light component is rendered lower when theangular component is great. For the reasons mentioned, there occurs aphenomenon that the black level rises while the contrast is considerablylowered.

Thus, in practical use, there exists a problem of trade-off (difficultyfor compatibility) between the luminance and the contrast. Because ofthis problem, in any conventional projection system equipped with such aknown device, there is employed an optical unit where the F number isgreater than 3.5 (more specifically, the F number of the projection lens5 or that of the illumination optical unit). That is, in any projectionoptical system equipped with the known device, the F number is notsettable to less than 3.5 due to a demand for practically realizing ahigh contrast to a certain degree, hence causing a failure in raisingthe luminance.

It is therefore a first object of the present invention to provideimprovements in a vertically-aligned liquid crystal display device whichis represented by a reflex liquid crystal display device of theinvention having a high response speed, wherein the liquid crystaltransmissivity reaches saturation at a low voltage despite a smallthickness of the liquid crystal layer, and the device can be driven withfacility on a driving circuit substrate manufacturable by an ordinarywithstand voltage process even in a small pixel size. The aboveimprovements also connote a display apparatus, a projection opticalsystem and a projection display system using such a reflex liquidcrystal display device of the invention.

A second object of the present invention resides in providing aprojection optical system and a projection display system where asufficiently low black level can be maintained in addition to the aboveaccomplishment even in a high-luminance optical unit having a small Fnumber, hence achieving a practically high contrast (i.e., meeting therequirements for both a higher luminance and a higher contrast incomparison with those of any conventional system).

SUMMARY OF THE INVENTION.

More specifically, the reflex liquid crystal display device of thepresent invention is such that a first substrate having a lighttransmissive electrode and a second substrate having a light reflectiveelectrode are positioned opposite to each other in a state where thelight transmissive electrode and the light reflective electrode aremutually opposed while a vertically-aligned liquid crystal layer isinterposed therebetween. In this display device, the vertically-alignedliquid crystal layer has a thickness of less than 2 μm, and thevertically-aligned liquid crystal material has a refractive indexanisotropy Δn of more than 0.1. Here, the definition of “lightreflective electrode” signifies an electrode being reflective itself tolight, an electrode having a light reflective layer thereon, and also anelectrode which may be transmissive itself to light but has an undercoatfilm on condition that light reflectivity is effected in the interfacebetween the electrode and the undercoat film.

The present invention relates also to a display apparatus equipped withthe reflex liquid crystal display device of the invention, and furtherto a projection optical system where the reflex liquid crystal displaydevice is disposed in its optical path, and a projection display systemusing such an optical system.

According to the present invention, although the vertically-alignedliquid crystal layer is less than 2 μm in thickness, the value Δn of thevertically-aligned liquid crystal material is adjusted to be more than0.1 differently from the conventional recognition, so that thetransmissivity of the liquid crystal reaches its saturation withfacility at a voltage lower than 5–6V, hence enabling satisfactorydriving at a practically low voltage and much enhancing thetransmissivity itself. Consequently, it becomes possible to achieveimprovements in the reflex vertically-aligned liquid crystal displaydevice having a sufficient transmissivity and superior drivingcharacteristic with low-voltage driving (low required withstand voltage)while holding a high response speed, and also in a display apparatus, aprojection optical system and a projection display system using suchimproved display device.

The remarkable advantageous functions and effects mentioned above areobtainable due particularly to the selective use of a vertically-alignedliquid crystal material having a value Δn of more than 0.1. In case theliquid crystal layer is reduced in thickness to less than 2 μm forattaining a high response speed, if the directional change of thedirector is to be affected by the interaction between the orientationfilm and the liquid crystal molecules, the incident light is prone to bepolarized and modulated, since Δn is greater than 0.1, in the liquidcrystal in compliance with the applied voltage to eventually cause readyseparation of the polarized light, whereby the desired transmissivitycan be obtained even at a low voltage.

The present invention also provides a projection optical system wherethe reflex liquid crystal display device of the invention and an opticalunit having an F number of less than 3 are disposed in its optical path,and further provides a projection display system using such an opticalsystem.

According to the above systems, a black level supposed to beproportional to the square of the thickness of the liquid crystal layercan be kept low as the thickness of the vertically-aligned liquidcrystal layer is set to less than 2 μm, and therefore a high contrastcan be realized even if the F number of the optical unit is less than 3,and yet a high luminance is also attainable with such a small F number.Thus, the projection optical and display systems, each of which isequipped with the reflex liquid crystal device of the invention and anoptical unit having an F number under 3, satisfy the requirements for ahigher contrast and a higher luminance in comparison with those of anyconventional system using the known device and optical unit. The Fnumber of the optical unit is controllable by the focal distance and soforth of a lens used therein.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 graphically shows V-T curves obtained by changing the refractiveindex anisotropy Δn of a vertically-aligned liquid crystal material in areflex liquid crystal display device (where the thickness d of a liquidcrystal layer is 2 μm);

FIG. 2 graphically shows V-T curves obtained by changing the refractiveindex anisotropy Δn of a vertically-aligned liquid crystal material in areflex liquid crystal display device (where the thickness d of a liquidcrystal layer is 1.5 μm);

FIG. 3 graphically shows V-T curves obtained by changing the refractiveindex anisotropy Δn of a vertically-aligned liquid crystal material in areflex liquid crystal display device (where the thickness d of a liquidcrystal layer is 1 μm);

FIG. 4 graphically shows the response speeds of a reflexvertically-aligned liquid crystal display device (where samples being 3μm and 3.5 μm thick indicate values of known devices);

FIG. 5 is a table of data representing the saturation voltage,transmissivity and response speed of each sample obtained in relation tothe thickness d, refractive index anisotropy Δn and permittivityanisotropy Δε of a vertically-aligned liquid crystal material in areflex liquid crystal display device;

FIG. 6 graphically shows comparative changes of the saturation voltagewith the refractive index anisotropy Δn of the same liquid crystal inrelation to the thickness d of the liquid crystal layer;

FIG. 7 graphically shows V-T curves obtained when the thickness of thesame liquid crystal layer is 3.5 μm and the refractive index anisotropyΔn of the liquid crystal is 0.13;

FIG. 8 graphically shows the dependency of the black-statetransmissivity on the thickness of the same liquid crystal layer (incomparison with the black-state value as 100% of a liquid crystal layerbeing 3.5 μm thick in a known device);

FIG. 9 graphically shows the black level changes caused in the reflexvertically-aligned liquid crystal device of the present invention incomparison with the black level changes relative to the F number of ameasuring optical unit in the known device;

FIG. 10 graphically shows the luminance changes caused in the sameliquid crystal device relative to the F number;

FIG. 11 is a schematic sectional view of the reflex vertically-alignedliquid crystal display device of the present invention;

FIG. 12 is a sectional view of principal portions on a driving circuitsubstrate of silicon in the display device of the invention;

FIG. 13 is an equivalent circuit diagram with the layout of the displaydevice of the invention;

FIG. 14 graphically shows a V-T curve of the known device (where thethickness of its liquid crystal layer is approximately 3 μm);

FIG. 15 graphically shows V-T curves obtained while reducing thethickness of the liquid crystal layer in the known device (whereΔn=0.082);

FIG. 16 graphically shows the changes caused in the saturation voltagein accordance with the thickness of the same liquid crystal layer; and

FIG. 17 is a schematic diagram of a projection optical system using theknown reflex liquid crystal display device.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS.

In the reflex liquid crystal display device of the present invention,the thickness of the vertically-aligned liquid crystal layer needs to beequal to or less than 2 μm for achieving the functional effectsmentioned above. It is more preferred that the thickness be in a rangeof 0.8 to 2 μm, and further in a range of 1 to 2 μm. Although theresponse speed is raised with a reduction of the layer thickness, thelower limit of the thickness is preferably 0.8 μm, and more preferably 1μm in regard to suppression of the interaction to the orientation filmand also in regard to controllability of the layer thickness. While thethickness of the liquid crystal layer may be small, Δn needs to begreater than 0.1 in order to enhance the polarized light separation, butan excessive increase of Δn is not exactly efficient for enhancing theeffect or is not practical either. Therefore, Δn may preferably be lessthan 0.25.

In a preferred structure, a liquid crystal orientation film is formed onthe opposed face of a transparent electrode of ITO (indium tin oxide) orthe like as the aforementioned light transmissive electrode and also onthe opposed face of the light reflective electrode of aluminum or thelike, and the light reflective electrode is connected to a singlecrystal semiconductor driving circuit of silicon or the like provided onthe aforementioned second substrate, thereby constituting an activedriving type. If a driving circuit substrate of silicon is employed asthe second substrate, the substrate itself is opaque and adapted forreflex type. Moreover, a MOS (metal oxide semiconductor) transistor as adriving element and an auxiliary capacity for voltage supply are suitedfor high-density integration attained with a minute pattern by thesemiconductor processing technology, so that it becomes possible torealize a high aperture rate, a high resolution due to enhancement of apixel density, reduction of a cell size, and enhancement of a carriertransfer rate.

Actually, the driving circuit comprises a driving transistor such asMOSFET (metal oxide semiconductor field effect transistor) provided foreach pixel on the silicon substrate, and the light reflective electrodeis connected to the output side of the driving transistor. The pixelsize can be reduced to equal to or less than 10 μm due to the use of alow withstand voltage transistor which is drivable at a low voltage. Andthe liquid crystal display device is also reducible in size to equal toor less than 2 inches diagonally.

Orientation control of the vertically orientated liquid crystal materialmay preferably be performed by means of a liquid crystal orientationfilm composed of a silicon oxide film. Such an orientation film can beformed by vacuum evaporation or the like with directivity (i.e., capableof easily controlling the pretilt angle of liquid crystal molecules).

In a display apparatus equipped with the reflex liquid crystal displaydevice of the present invention and also in projection optical anddisplay systems where such liquid crystal display device is disposed inthe optical path thereof (further with an optical unit having an Fnumber under 3), a light source and an optical unit for enablingincidence of the light from the light source onto the reflex liquidcrystal display device may preferably be disposed in the optical pathtogether with the reflex liquid crystal display device and anotheroptical unit for introducing the reflected light from the reflex liquidcrystal display device.

In this case, it is preferred that the light emitted from the lightsource is incident upon the reflex liquid crystal display device via apolarizer/converter and a polarized beam splitter, and the reflectedlight from the reflex liquid crystal display device is introduced viathe polarized beam splitter again or is introduced further via aprojection lens to a screen.

It is also preferred that the reflex liquid crystal display device andthe polarized beam splitter are disposed for each of colors, and thereflected light components from the individual reflex liquid crystaldisplay devices are synthesized or are introduced further to theprojection lens. More concretely, white light emitted from a white lightsource is introduced via the polarizer/converter to a dichroic colorseparating filter, which then separates the light into respective lightcomponents of the individual colors. Subsequently, the light componentsare incident upon the reflex liquid crystal display devices respectivelyvia the polarized beam splitter, and the reflected light componentstherefrom are synthesized by means of a prism.

Here, the F number of the optical unit used in combination with thereflex liquid crystal display device of the invention needs to be asmall value of less than 3 for attaining compatibility of a highcontrast and a high luminance. Preferably, however, the F number isdesired to be not more than 3.0 and not less than 1.5 (further not lessthan 2.0) in order to enhance the effect.

Now a preferred embodiment of the present invention will be describedbelow in detail with reference to the accompanying drawings.

First, FIG. 11 shows the fundamental configuration of a liquid crystalelectro-optical device constituting a display apparatus which isrepresented by the preferred embodiment.

This device serving as a reflex liquid crystal display device 23comprises a silicon driving circuit substrate 31 composed of singlecrystal of silicon or the like and having a light reflective electrode30 of a pixel structure, and a transparent substrate 33 of glass or thelike having a transparent electrode 32 and positioned opposite to thesubstrate 31, wherein a vertically-aligned liquid crystal 36 is sealedbetween the two substrates (actually between liquid crystal orientationfilms 34 and 35). As shown in FIG. 12, a reflective electrode substrateserving as the driving circuit substrate is such that a driving circuitcomprising CMOS and n-channel MOS transistors Tr and capacitors C isformed on a single crystal silicon substrate 37, and a light reflectiveelectrode 30 of a pixel structure is formed thereon with a metal film ofaluminum, silver or the like. In case the light reflective electrode iscomposed of metal such as aluminum, it functions as both a lightreflecting film and an electrode to apply a voltage to the liquidcrystal. For the purpose of further raising the light reflectivity, alight reflective layer with a multi-layer film such as a dielectricmirror may also be formed on the aluminum electrode.

In FIG. 12, the transistor Tr comprises, for example, an n-type sourceregion 38, a drain region 39, a gate insulating film 40 and a gateelectrode 41, wherein electrodes 42 and 43 are led out from the activeregions respectively. In this structure, the electrode 43 is connectedvia an inter-layer insulating film 47 to a capacitor electrode 46 whichis in contact with an insulating film (dielectric film) 45 on an n-typeregion 44 constituting a capacitor C. The electrode 43 is connected alsoto a wire 50 via inter-layer insulating films 48, 49 and further to thelight reflective electrode 30. In this device, the s-polarized incidentlight 10(s) shown in FIG. 17 is converted in accordance with the appliedvoltage in the layer of the vertically-aligned liquid crystal 36,whereby reflected light 10(p) including p-polarized light is obtained,and then the light 10(p) is introduced to the aforementioned polarizedbeam splitter 2.

In the reflex liquid crystal display device of the present invention,the layer thickness d (cell gap) of the vertically-aligned liquidcrystal 36 is set to be equal to or less than 2 μm, and the refractiveindex anisotropy Δn of the vertically-aligned liquid crystal 36 employedhere is more than 0.1.

FIG. 13 shows a fundamental layout of the display device and anequivalent circuit of its pixel portion. The silicon driving circuitsubstrate 31 comprises a pixel driving circuit formed in each pixel, anda logic driver circuit (data driver, scanning driver and so forth)incorporated in the periphery of a display area. The pixel drivingcircuit formed under each light reflective (pixel) electrode 30 consistsof a switching transistor Tr and an auxiliary capacitance C forsupplying a voltage to the vertically-aligned liquid crystal 36. Thetransistor Tr is required to withstand a predetermined voltagecorresponding to the driving voltage for the vertically-aligned liquidcrystal, and it is produced normally by a higher withstand voltageprocess as compared with the logic. Since the transistor size becomesgreater with a rise of the withstand voltage, usually a transistorhaving a withstand voltage of 8 to 12V or so is used in view of theproduction cost and the power consumption. Therefore, it is desired todesign that the liquid crystal driving voltage is set within ±6V. Thisrequirement can be met according to the present invention.

In the vertically-aligned liquid crystal 36 used in this display device,each molecule is so oriented that the major axis thereof is renderedsubstantially vertical to the substrate when no voltage is applied, andupon application of a voltage, the major axis is inclined to thein-plane direction to thereby change the transmissivity. If theinclinations of the liquid crystal molecules are not the samedirectionally when the liquid crystal is driven, there occurs somenon-uniformity in brightness and darkness. In order to avoid such adisadvantage, it is necessary to vertically orient the liquid crystal bypreviously giving a slight pretilt angle in a fixed direction (generallyin the diagonal direction of the device) as shown in FIG. 11.

If the pretilt angle is excessively large, the vertical orientationcharacteristic is deteriorated with a rise of the black level toeventually lower the contrast while affecting the V-T curve. Therefore,the pretilt angle is controlled generally within a range of 1° to 7°.Each of the liquid crystal orientation films 34 and 35 to be given sucha pretilt angle is composed of a silicon oxide film represented by SiO₂,such as an oblique evaporated film, or a polyimide film. In the former,the evaporation angle given at the time of oblique evaporation is in arange of 45° to 55°; meanwhile in the latter, the pretilt angle iscontrolled within a range of 1° to 7° by changing the rubbing condition.

In the known device, the thickness d of the vertically-aligned liquidcrystal layer in the device structure of FIG. 11 is approximately 3 to 4μm, and there is used a selected liquid crystal material where therefractive index anisotropy Δn is less than 0.1 (typically 0.08 or so).However, if the thickness d of the liquid crystal layer in the knowndevice is reduced to less than 2.5 μm, the response speed is renderedhigher but the driving voltage is raised as described above, so that therequirements for practical use fail to be satisfied. The mechanism ofthis phenomenon that the driving voltage is raised with reduction of thethickness of the liquid crystal layer is not exactly definite, but it isconsidered to be derived from that, while the bulk property of theliquid crystal appears principally with an increase of the layerthickness, the influence of the interaction on the interface between theorientation film and the liquid crystal is not negligible (i.e., theinteraction is supposedly so exerted as not to incline the liquidcrystal molecules).

As the result of repeating many experiments in order to overcome theproblems mentioned above, the inventor has found that the problems canbe solved by selectively controlling the refractive index anisotropy Δnof the vertically-aligned liquid crystal material to more than 0.1.FIGS. 1 and 2 graphically show V-T curves obtained by changing theanisotropy Δn of the liquid crystal under conditions that the thicknessd of the liquid crystal layer is 2 μm and 1.5 μm, respectively. It isseen from these diagrams that, despite reduction of the thickness d ofthe liquid crystal layer to less than 2 μm in particular, thetransmissivity is easily saturated at a low voltage of 4 to 6V or lessif the anisotropy Δn is held over 0.1, whereby practical driving isachievable.

According to the present invention, even in a display device where thethickness d of its liquid crystal layer is extremely small as 1 μm, thetransmissivity is saturated at a low driving voltage of 6V or so if theanisotropy Δn is held over 0.1, as shown in FIG. 3. It is also seen thatremarkable improvements can be attained in comparison with anyconventional device where the transmissivity obtained by using the knownmaterial composition is merely 30% or so. Particularly due to the use ofa selected liquid crystal material having a high value of Δn=0.13, it ispossible to realize, even with a thickness of 1 μm, an excellent reflexdisplay device which uses vertically-aligned liquid crystal of siliconand indicates a sufficient transmissivity with superior drivingcharacteristic.

FIG. 4 graphically shows the response speed (rise time+fall time) of thereflex liquid crystal display device according to the present invention.As plotted, the response is much faster in comparison with that in anyconventional device, such as 7 to 9 msec with d=2 μm, or under severalmsec with d=1.5 μm or less. (However, with d=2.5 μm, the response speedis lowered to 13–14 msec.) In the device with d=1.5 μm or less, the fastresponse is kept under 8 msec even in a half tone. This device iscapable of realizing a satisfactory image quality even in motionpictures of movies or television pictures where half-tone display isfrequently employed with many moving images.

FIG. 5 is a table showing the characteristics of the display device(samples Nos. 7–15) of the present invention and those of comparativeexamples (samples Nos. 1–6, 16–19). FIG. 6 graphically shows changes ofa saturation voltage with Δn in relation to the thickness d of a liquidcrystal layer. In view of the driving characteristic, the transmissivityand the response speed, suitable values adapted for practical use are asfollows. The thickness d of the liquid crystal layer is preferably lessthan 2 μm, and particularly 1 to 2 μm; Δn of the liquid crystal with d=2 μm is preferably Δn≧0.1 (more preferably Δn≧0.103, further preferablyΔn≧0.114); with d=1.5 μm, Δn≧0.106 (more preferably Δn≧0.11, furtherpreferably Δn≧0.114); and with d=1 μm, Δn≧0.104 (more preferablyΔn≧0.114, further preferably Δn≧0.12).

FIG. 7 graphically shows V-T curves obtained when the thickness of theliquid crystal layer in the known device is 3.5 μm by the use of avertically-aligned liquid crystal material having a high refractiveindex anisotropy Δn of more than 0.1, i.e., in the case of Δn=0.13 forexample. As seen from this graph, the threshold voltage is considerablylowered, and saturation is attained at a driving voltage ofapproximately 2V. However, as obvious from the aforementioned Eq. (1),the response speed is in inverse proportion to the square of the drivingvoltage while being changed in conformity with the thickness d of theliquid crystal layer, so that such a low driving voltage extremelydeteriorates the response speed. According to the results of actualmeasurements, the black-and-white response speed of this device is 46msec (approx. 50 msec), and in a half tone, the response speed islowered to 100 msec or so due to a further drop of the driving voltage,hence causing manifest difficulty for practical use. Thus, in the knowndevice, it is necessary to reduce the value of Δn under 0.1 in view ofthe response speed.

As described above, the present invention has been accomplished by newlyfinding the requisite value of Δn of the liquid crystal material so asto realize an improved reflex vertically-aligned liquid crystal devicewhere the thickness d of its liquid crystal layer is less than 2 μm.Therefore, even if the thickness d of the liquid crystal layer is lessthan 2 μm, the saturation voltage can be lowered by selectivelyadjusting the refractive index anisotropy as Δn≧0.1, hence enhancing theresponse speed as well.

The table shown below gives the values of Δn (also those of Δε) ofvertically-aligned liquid crystal materials (made by Merck Ltd.,).

Vertically-aligned liquid crystal material Sample A Sample B Sample CSample D Δn +0.082 +0.103 +0.114 +0.13 n (∥) 1.557 1.584 1.598 1.62 n(⊥) 1.475 1.481 1.484 1.49 Δ ε −4.1 −0.5 −5.3 −4.3 ε (∥) 3.5 4.0 3.9 3.8ε (⊥) 7.6 9.0 9.2 8.1

Next, a description will be given on the advantage that thevertically-aligned liquid crystal display device of the invention ismore effective for an optical unit of a smaller F number as comparedwith any known device.

First, it has been found that the black level in the device of theinvention having a thinner liquid crystal layer can be lowered under theblack level obtained in the known device where the liquid crystal layerhas a thickness of 3 to 4 μm. In FIG. 8, each black level value(black-state transmissivity at zero voltage) in the vertically-alignedliquid crystal display device of the invention is graphically shown as afunction of the thickness of the liquid crystal layer. In the respectivematerials, the numerical values obtained with a layer thickness of 3.5μm are expressed as 100% (where the abscissa represents the thickness ofthe liquid crystal layer).

When the applied voltage is zero, the liquid crystal molecules areoriented to be substantially vertical to the substrate plane, so that inprinciple the incident light is reflected without any change of thepolarized state and then is returned to the incidence side by means of apolarized beam splitter. However, in the actual device, the liquidcrystal molecules are inclined at a pretilt angle and are thereforerendered slightly elliptical, and moreover the light separationcharacteristic of the polarized beam splitter is dependent on theincidence angle as mentioned, whereby the black-state transmissivity israised to consequently deteriorate the contrast.

Meanwhile in the display device of the present invention, it has beenfound that the black-state transmissivity is lowered with a reduction ofthe thickness of the liquid crystal layer and, as shown in FIG. 8, theblack level value obtained with a layer thickness of 2 μm becomes 20–30%as compared with the value in the known device, or 10–20% with a layerthickness of 1.5 μm, or 5–15% with a layer thickness of 1.0 μm. (Butwhen the layer thickness is 2.5 μm, the black level value becomes highas 40–50%). Regarding the contrast which is expressed by the ratio ofwhite and black levels, since the white level is kept substantiallyunchanged, the result shown in FIG. 8 indicates that the contrastattained in the device of the present invention becomes higher than fiveto ten times or more with a layer thickness of 1.5 μm for example.

Such fall of the black level value with a reduction of the thickness ofthe liquid crystal layer is considered to be based principally on thefollowing reasons. The transmissivity T of the liquid crystal in thedevice of the present invention is expressed by Eq. (4).T∝ sin²(2d·Δn(eff)·π/λ)  (4)

In the above, λ denotes the wavelength of the light, and Δn(eff) denotesthe effective refractive index anisotropy corresponding to theinclination angle θ from the perpendicular direction of liquid crystalmolecules. This anisotropy is expressed by Eq. (5).

$\begin{matrix}{{\Delta\;{n({eff})}} = {\frac{{n( {\mspace{14mu} } )}{n(\bot)}}{\sqrt{\lbrack {{{n( {\mspace{14mu} } )}^{2} \cdot {\cos^{2}(\theta)}} + {{n(\bot)}^{2} \cdot {\sin^{2}(\theta)}}} \rbrack}} - {n(\bot)}}} & (5)\end{matrix}$

The inclination angle θ of the liquid crystal molecules is widened witha rise of the liquid crystal driving voltage, and Δn(eff) is increasedcorrespondingly thereto to raise the transmissivity consequently. It isseen that, when θ=90° in principle, Δn(eff) becomes equal to the valueof Δn of the liquid crystal material. According to Eq. (4), thetransmissivity T becomes 100% when the condition of 2d·Δn(eff)·π/λ=π/2is satisfied. a black state, becomes zero if the liquid crystalmolecules are oriented completely vertically as (θ=0), so thatΔn(eff)=0. Actually, however, the liquid crystal molecules are orientedwith a pretilt angle of 1 to 7° as mentioned, whereby the value ofΔn(eff) is rendered finite to consequently give the black-statetransmissivity. As the black-state transmissivity is raised with anincrease of the pretilt angle, it is preferred that the pretilt angle becontrolled to less than 5°. Since 2d·Δn(eff)·π/λ has a small value atthe black level, Eq. (4) may be rewritten approximately asT∝ sin²(2d·Δn(eff)·π/λ)≈(2d·Δn(eff)·π/λ)^(2.)Therefore, T is theoretically considered to be proportional to thesquare of the thickness d of the liquid crystal layer. The data of FIG.8 obtained from the actual measurement can be explained substantially inaccordance with this relation.

Thus, the thickness d of the liquid crystal layer in this device is sodesigned as to be less than 2 μm, and therefore the black level can besuppressed low essentially in comparison the known device where thelayer thickness is 3 to 4 μm, hence realizing a high contrast.

If the F number of the optical unit is decreased in the known device,the black level is raised to eventually fail in ensuring a desiredcontrast, and therefore it is unavoidable to set the F number forcedlyto more than 3.5, as already described. However, in the device of thepresent invention, the black level in the device itself is heldextremely low as explained, so that a sufficiently high contrast can beensured even in the optical unit having a small F number.

FIG. 9 graphically shows changes caused in the black-statetransmissivity by changing the F number of the projection lens 5 in FIG.17 and that of the measuring optical unit corresponding to theillumination optical unit. The black level rises with a decrease of theF number, but in the device of the present invention, the black level iskept lower than in the known device at any F number, whereby asufficiently high contrast can be realized even in the optical unithaving a smaller F number under 3. Moreover, a satisfactory highluminance is still attained with an F number of less than 3, as shown inFIG. 10. (However, the luminance is saturated when the F number is under2). And the luminance is considerably lowered if the F number exceeds 3.

Regarding the luminance, it has been found experimentally that, in apractical projection system with an optical unit using a 120 W lamp in adiagonally 0.7-inch device for example, the luminance is enhancedapproximately 60% when the F number is changed from 3.85 to 2.

As mentioned above, a superior projection system, which is capable ofmeeting the requirements for both a higher contrast and a higherluminance in comparison with any known system using the conventionaldevice and optical unit, can be provided due to the display device ofthe present invention and also to a projection optical system and aprojection display system each employing an optical unit of an F numberunder 3.

Hereinafter the preferred embodiment of the present invention will bedescribed more specifically with some comparative examples.

COMPARATIVE EXAMPLE 1

Each conventional known device was produced as follows. First, a glasssubstrate with a transparent electrode and a driving circuit substrateof silicon with an aluminum electrode were washed and then wereintroduced into an evaporator, where a liquid crystal orientation filmof SiO₂ was formed by oblique evaporation in an angular range of 45 to55°. The thickness of the liquid crystal orientation film was set to 50nm, and the pretilt angle of the liquid crystal was so controlled as tobe approximately 2.5°.

Thereafter an adequate number of glass beads having a diameter of 1 to3.5 μm were sprinkled between the two substrates where the liquidcrystal orientation film was formed, and the two substrates were joinedtogether. Subsequently, a vertically-aligned liquid crystal material(made by Merck Ltd.,) having a negative permittivity anisotropy Δε and arefractive index anisotropy Δn of 0.082 was injected between thesubstrates to thereby produce six kinds of reflex liquid crystal displaydevices (samples Nos. 1–6 in FIG. 5) where the liquid crystal layerthickness (cell gap) was 3.5 μm, 2.9 μm, 2.5 μm, 2 μm, 1.5 μm and 1 μmrespectively.

In each of the devices thus produced, a voltage was applied between thetransparent electrode and the aluminum electrode, and the changes causedin the transmissivity of the liquid crystal by changing the appliedvoltage were measured. (Since the devices are of reflex type, actuallythe reflectivity thereof is measured. However, measuring thereflectivity is equivalent to measuring the transmissivity of the liquidcrystal, and therefore it will be so described below.) The measurementwas performed at room temperature.

FIG. 15 graphically shows the liquid crystal driving characteristics ofsuch devices. As shown in FIGS. 15 and 16, the saturation drivingvoltage rises sharply over 6V when the thickness of the liquid crystallayer is less than 2.5 μm.

[Embodiment]1

In the same method as adopted for Comparative example 1 mentioned above,a liquid crystal orientation film of SiO₂ was formed on each of asubstrate with a transparent electrode and a driving circuit substrateof silicon with an aluminum electrode, and three kinds ofvertically-aligned liquid crystal materials (made by Merck Ltd.,) havinga negative permittivity anisotropy Δε and a refractive index anisotropyΔn of 0.103, 0.114 and 0.13 were injected between the two substrates tothereby produce nine kinds of reflex liquid crystal display devices(samples Nos. 7–15 in FIG. 5) where the liquid crystal layer thickness(cell gap) was 2 μm, 1.5 μm and 1 μm respectively. The pretilt angle ofthe liquid crystal was so controlled as to be approximately 2.5°.

The liquid crystal driving characteristics of the devices thus producedwere measured at room temperature similarly to Comparative example 1.FIGS. 1, 2 and 3 graphically show the driving characteristics obtainedin three cases where the thickness of the liquid crystal layer is 2 μm,1.5 μm and 1 μm respectively. FIG. 5 is a table showing the drivingvoltages of the individual devices at which the transmissivity issaturated substantially, and also the respective values of thetransmissivity.

It is seen from such results that, as Δn is controlled to be more than0.1, the transmissivity is saturated easily at a low voltage of 4 to 6Vdespite reduction of the liquid crystal layer thickness d under 2 μm,whereby practical driving can be performed. Furthermore, since thetransmissivity is much enhanced in comparison with any conventionaldevice, it becomes possible to realize an improved silicon reflex typevertically-aligned liquid crystal display device having a sufficienttransmissivity and superior driving characteristic.

Other devices were also produced by forming, as a liquid crystalorientation film, a polyimide film instead of a silicon dioxide film,and controlling the orientation by rubbing. The results were the same asthose mentioned above.

[Embodiment]2

The response speed relative to a rise time (from black to white) and afall time (from white to black) was measured in each of the reflexliquid crystal display devices produced in Embodiment 1. The sum totalthereof is regarded as the response speed of each device, and the resultis shown in FIG. 5. The measurement was performed at room temperature.FIG. 4 graphically shows the thickness d of the liquid crystal layer asa function with regard to the device having Δn=0.13 as a representativeexample (samples Nos. 9, 12, 15 with d=2.5 μm in FIG. 5). Forcomparison, FIG. 4 also shows the response speeds of the known sampleNo. 1 and the sample produced with d=3 μm (Δn=0.082 in each sample).

As supposed from Eqs. (1) and (2), the response speed changessubstantially in proportion to the square of the thickness of the liquidcrystal layer. In the device of the present invention where the layerthickness d is less than 2 μm and Δn is more than 0.1, it was proved torealize a high-speed response faster than 9 msec.

COMPARATIVE EXAMPLE 2

In the same method as adopted for Embodiment 1, a reflex liquid crystaldevice (sample No. 16) was produced by using a liquid crystal materialof Δn=0.13 with a layer thickness of 3.5 μm, and the liquid crystaldriving characteristic was measured.

FIG. 7 graphically shows the result in comparison with thecharacteristic of sample No. 1 obtained with Δn=0.082. As shown, thedriving voltage in the device (sample No. 16) was much lower. Theresponse speed measured at room temperature in the same manner as inEmbodiment 2 was 46 msec. Since the driving voltage in a half tone islow as 1V or so, the response speed was rendered lower, and in agray-scale gradation of 25%, the response speed was further lowered tothe vicinity of 100 msec.

[Embodiment 3]

The transmissivity (black level) at a zero applied voltage (black state)was measured in the reflex liquid crystal display device produced inEmbodiment 1. For systematically examining the black level changescaused in relation to the thickness of the liquid crystal layer, deviceshaving a layer thickness of 3.5 μm (samples Nos. 17–19) were producedwith the aforementioned samples of Δn, and also devices having a layerthickness of 2.5 μm were produced similarly, and the black-leveltransmissivity of each device was measured together with the samples(Nos. 7–15) of Embodiment 1. The respective black level values aregraphically shown in FIG. 8 where the numerical values obtained in thedevices with a layer thickness of 3.5 μm are indicated as 100% with theindividual samples of Δn.

As shown in FIG. 8, the black level is extremely lowered when the liquidcrystal layer becomes thinner than 2 μm in any sample of Δn. In thedevice with a layer thickness of 1.5 μm for example, the indicated blacklevel is lower by 10 to 20% than the value obtained in any device with alayer thickness of 3.5 μm. That is, the contrast of the device becomesso high as 5 to 10 times. In the measuring optical unit of FIG. 7 havingan F number of 3.85, this trend remained substantially the same despiteany change of the F number.

[Embodiment 4]

The devices (samples Nos. 12, 9) of Embodiment 1 with Δn=0.13 and aliquid crystal layer thickness of 1.5 am and 2.0 μm were incorporated inthe measuring optical unit having an F number of 3.85, 3 and 2, and theblack level (black-state transmissivity) of each device was comparedwith that of the known device (sample No. 1).

FIG. 9 graphically shows the results of such comparison. The black levelrises with a decrease of the F number. However, in the device of thepresent invention, the black level is maintained lower than that in theknown device despite any change of the F number. The white leveltransmissivity in each device was kept substantially unchanged at 0.6 orso. Therefore, the black level ratio directly gives the contrast ratioof the device. According to the device of the invention, it is seen thatin any optical unit having a small F number under 3, an equal or highercontrast can be realized as compared with the known device. The lowerlimit of the F number may be set preferably to 1.5, further preferablyto 2.0.

In conformity with the specification described above, a diagonally0.7-inch silicon reflex type vertically-aligned liquid crystal displaydevice was produced, and the luminance obtained by the use of a 120 Wlamp as a light source was compared in a practical projection opticalunit having an F number of 3.85, 3.5, 3, 2.5 and 2. The results aregraphically shown in FIG. 10 where, as compared with the luminanceobtained in an optical unit of F number=3.85, the luminance is enhancedabout 32% with F number=3, about 44% with F number=2.5, about 60% with Fnumber=2, and sharply enhanced with F number≦3. However, the luminancewas enhanced merely 15% or so with F number=3.5, or not changedsubstantially with F number=1.5 as compared with the value obtained withF number=2. Regarding the contrast, even in an optical unit with Fnumber≦3, the contrast attained was higher than the value in the knowndevice, as mentioned. That is, a superior projection system was realizedto meet the requirements for both a higher luminance and a highercontrast in comparison with those in the known device.

It is to be understood that the embodiments and examples of the presentinvention described above may be modified variously on the basis of thetechnical concept of the invention.

For instance, the structure, material and so forth of the componentparts of the reflex liquid crystal display device or those of an opticalor projection system equipped with such display device are not limitedmerely to the aforementioned ones alone, and may be altered with avariety of modifications.

Thus, according to the present invention where the Δn of thevertically-aligned liquid crystal material is controlled to be more than0.1, the transmissivity of the liquid crystal is saturated with facilityat a low voltage below 5 to 6V despite a reduction of the thickness ofthe vertically-aligned liquid crystal layer to less than 2 μm, henceachieving satisfactory driving at a practically low voltage whileattaining another advantage of remarkable improvement in thetransmissivity itself. Therefore, it becomes possible to realize asuperior reflex type vertically-aligned liquid crystal display devicewhich indicates a sufficient transmissivity, an excellent low-voltagedriving characteristic (low required withstand voltage) and fastresponse as well. Further improvements are realizable in a displayapparatus, a projection optical system and a projection display systemby the use of such a display device.

In these systems where the thickness of the vertically-aligned liquidcrystal layer is reduced to less than 2 μm, the black level, which isconsidered to be proportional to the square of the thickness of theliquid crystal layer, can be held low to thereby achieve a high contrasteven when the F number of the optical unit is less than 3, and a highluminance is also achievable with a small F number. Consequently, itbecomes possible to provide a superior system which is capable ofsatisfying the requirements for both a high contrast and a highluminance.

1. A reflex liquid crystal display device comprising: a first substratewith a transparent electrode; a second substrate with a light reflectiveelectrode; and a layer of vertically-aligned liquid crystal materialinterposed between said first and second substrates positioned oppositeto each other in a state where said transparent electrode and said lightreflective electrode have mutually opposed surfaces, wherein analignment film is formed on each of the mutually opposed surfaces ofsaid transparent electrode and said light reflective electrode, and thethickness of said vertically-aligned liquid crystal layer is less than 2μm, the refractive index anisotropy Δn of said liquid crystal materialis more than 0.1 and less than 0.13, and said vertically-aligned liquidcrystal layer includes a plurality of liquid crystal molecules eachhaving a pre-tilt angle controlled within a range of 1° to 7°, wherein avoltage for saturating a light output with respect to a light input tosaid layer is within a range of less than 6 volts.